Microwave balanced external cavity rejection filter

ABSTRACT

In an axially symmetrical waveguide, a balanced external cavity filter comprising a plurality of resonant cavity pairs coupled to the waveguide and disposed on the circumference of the same in a plane orthogonal to its axis, constitutes a novel mode-filter rejecting undesired polarizations in certain waveguide propagation-modes determined by the angular positions of the cavity pairs and their coupling to the waveguide at frequencies at which the cavity pairs are resonant.

United States Patent Bastikar 51 June 27, 1972 [54] MICROWAVE BALANCEDEXTERNAL CAVITY REJECTION FILTER [72] Inventor; Arvid RamkrishnaraoBastikar, Kanata,

Ontario, Canada Northern Electric Company Limited, Montreal, Quebec,Canada 221 Filed: April 5,1971

21 Appl.No.: 131,280

[73] Assignee:

[52] US. Cl. ..333/73 W, 333/10 [51] ....H03h 7/10, H03h 9/00 [58]FieldoiSearch ..333/10,73,73W

[56] References Cited UNITED STATES PATENTS 2,877,434 3/1959 Farr et al..333/98 M 3,302,111 l/l967 Jones et al... ....333/98 M 3,579,153 5/1971Wang ..333/73W 3,546,633 12/1960 Peddiatt ..333/73 W 2,785,381 3/1957Brown 333/98 M X 2,106,768 2/1938 Southworth. 333/73 W 3,496,497 2/ 1970Conning ..333/98 M 2,853,682 9/1958 Epstein... ..333/73 W X PrimaryExaminerEli Lieberman Assistant Examiner-Saxfield Chatmon, Jr.Attomey-S. T. Jelly ABSTRACT In an axially symmetrical waveguide, abalanced external cavity filter comprising a plurality of resonantcavity pairs coupled to the waveguide and disposed on the circumferenceof the same in a plane orthogonal to its axis, constitutes a novelmode-filter rejecting undesired polarizations in certain waveguidepropagation-modes determined by the angular positions of the cavitypairs and their coupling to the waveguide at frequencies at which thecavity pairs are resonant.

2 Claims, 3 Drawing Figures PATENTEDJIINZT 1912 3.673 .522

- SHEET 2 0r 2 MICROWAVE BALANCED EXTERNAL CAVITY REJECTION FILTER FIELDOF THE INVENTION This invention relates to microwave waveguide filtersand more particularly to balanced external cavity rejection filters foraxially symmetrical waveguides (referred to in the following disclosuresimply as symmetrical waveguides).

BACKGROUND OF THE INVENTION In the utilization of symmetricalwaveguides, such as circular waveguides, for the transmission of signalsor power at microwave frequencies, several problems arise. For example,because the cut-off frequency of the second mode of propagation TM in acircular waveguide is only approximately 31 percent higher than thecut-off frequency of the basic mode TE the region of stable transmissionin the basic mode is limited as compared to rectangular waveguide, wherethe second mode is approximately 100 percent higher than the basic mode.Also, the practical utilization of the TE basic mode is somewhatdifficult. Slight irregularities in the waveguide, e.g., slightelipticity, cause the TE wave to have two different phase and groupvelocities for each of its circularly polarized components.

In the authoritative text book titled Microwave Transmission Circuits,edited by George L. Ragan, Radiation Laboratories (MIT) Series, No. 9,published by McGraw-Hill Book Company, Inc., 1948, it is stated on page54, section 2.10: Round waveguide is seldom used as a transmission linebecause of difficulties arising from the fact that there is no uniquedirection of polarization of the fields. That is to say, the fieldpattern may be rotated through any angle about the axis of the tube.[i.e., the waveguide]. Imperfections in the tubing and bends required ininstalling it tend to introduce other components of polarization whichthen get out of phase, resulting in elliptical polarization of thewaves.

One of the interesting modes of propagation is the TE mode. This modepossesses only axially and radially magnetic components; the absence ofa tangential magnetic component causes attenuation to decrease withincreasing frequency. In order to achieve sufficient reduction inattenuation, however, one must go much higher in frequency than thebasic TE mode, leaving many other modes in between. To avoid theinstabilities arising thereby, novel mode-filters would be necessary.

DESCRIPTION OF THE PRIOR ART In a microwave antenna system, it isusually required that either a horizontally or vertically polarizedsignal be radiated, or eventually a circularly polarized signal. If themicrowave antenna is fed by a symmetrical waveguide, means forsuppressing the polarizations other than that to be radiated arenecessary. To achieve this, the practice has been to place fins" insidethe waveguide in certain known arrangements to attenuate unwanted modesand polarizations. These fins, however, cause losses in the wantedsignals themselves and excite spurious modes, because of the sometimessevere discontinuities the fins create. They are imperfections in thewaveguide leading to the complications mentioned by Ragan supra. Powerlosses in such cases, and these spurious modes and polarizations poseproblems, especially since an antenna feed system usually carries hightransmitter power. A restriction, therefore, was imposed on the numberof such fins, and hence on the effective rejection of unwanted modesthey provide.

SUMMARY OF THE INVENTION This invention obviates the generaldifficulties inherent in the use of circular waveguides by introducing afilter which does not utilize internal fins, and therefore overcomesdisadvantages of the prior art.

The novel filter is a balanced external cavity filter. The balancedpairs of resonant cavities are diametrically disposed on thecircumference of the symmetrical waveguide at predetermined angles withrespect to each other, and are all in the same orthogonal plane to thewaveguide axis. Depending on what frequencies and modes must berejected, each pair of cavities is made resonant in a well-known mannerat the rejected frequency, and the means of coupling each pair ofcavities to the waveguide are determined. In preferred embodiments,coupling irises in the wall of the waveguide, between the same and eachresonant cavity, are employed. The shape and/or orientation of irises,i.e., circular, rectangular, etc., and the angular position of theresonant cavity pairs with respect to each other, determines the mode ormodes rejected as shall be shown in describing the preferred embodimentof the invention.

This novel filter can be used with various types of symmetricalwaveguides such as: circular, octagonal, hexagonal, or square. Theresonant cavities can be cylindrical, rectangular, or such other formsof resonant cavities as may be expedient, and as are well known in theart.

The invention makes possible the simultaneous rejection of unwantedsignals in the dominant mode and higher modes, as well as of harmonics,circularly polarized signals, and simultaneously, enables polarizationdiscrimination of linearly polarized signals.

This type of filter can be used as a band-stop filter; in antenna feedsystems as a higher mode rejection filter; or as a linear polarizationdiscriminator.

In a preferred embodiment of the invention as a polarization and highermode rejection filter, three pairs of resonant cavities are utilized.The first of said pairs being oriented either vertically, if thehorizontal polarization is to be rejected and the vertical to beradiated, or it is oriented horizontally if the vertical polarization isto be rejected and the horizontal to be radiated. In general, theorientation is arbitrary if not specified by the application. This firstpair of cavities is being coupled inductively to the waveguide by meansof a pair of rectangular irises. The second and third pairs of resonantcavities are positioned one on each side of the first cavity pair and at45 with respect to it. In order to couple more than one mode to thesecond and third cavity pairs, namely TM TE TM and TM at the samefrequency, the coupling means may be circular irises. The positioning ofthese second and third cavity pairs at 45 angle with respect to thefirst pair enables the rejection of circularly polarized signals at thefrequencies at which the cavities are resonant and that are in theabove-mentioned modes.

In the above-described embodiment, three pairs of resonant cavities areefficiently utilized to offer a novel solution to the problem ofpolarization and higher mode rejection which may be used in an antennafeed system.

BRIEF DESCRIPTION THE DRAWINGS An example embodiment of the inventionwill now be described with reference to the accompanying drawings inwhich:

FIG. 1 is a perspective view of a balanced external cavity filter withthree cavity pairs disposed on a circular waveguide section;

FIG. 2 is a cross-section taken along the line ll--ll of the perspectiveview, FIG. 1; and

FIG. 3 is a development along the line III-Ill in the perspective view,FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIGS. 1, 2 and 3, thebalanced external cavity filter comprises a section of an axiallysymmetrical waveguide 10, first, second and third pairs of resonantcavities lla-l lb, 12a-l2and l3a-13b, respectively, each pair beingdiametrically disposed on the waveguide 10 in a plane orthogonal to itsaxis. The second and third pairs of resonant cavities l2al2b and -13!)respectively, being disposed at 45 with respect to the first pair ofresonant cavities Ila-11b, one on either side thereof. The first pair ofresonant cavities Ila-11b being couple inductively to the waveguidethrough rectangular irises 14a-14b. The second and third pairs ofresonant cavities 124-12 and l3a-13b, respectively, being coupled to thewaveguide 10 through circular irises a-l5b and 16a 16b, respectively.The first, second and third pairs of resonant cavities 11 a-llb,124-121: and l3a13b, respectively, being resonant at substantially thesame frequency.

The use of the preferred embodiment as described herein, in an antennafeed system is particularly interesting. Assuming an axially symmetricalwaveguide 10 feeding the microwave antenna (not shown) and carryingtransmitted power at a frequency in the TE mode launched in a verticalpolarization. Due to the lack of a unique direction of polarization inthe waveguide as stated by Ragan supra, it is required to redefine thepolarization originally launched. Therefore, the

first pair of resonant cavities 1 10-1 1b is made resonant at thetransmitted frequency, and is coupled inductively to the waveguide 10through the rectangular iris pair 14a-14b in a well known manner. Themagnetic field of the undesired polarization which is perpendicular tothe orientation of the resonant cavity pair Ila-11b (i.e., horizontal)is coupled to the pair Ila-11b and therefore rejected. The orientationof the resonant cavity pair Ila-11b, then, is that parallel to thelinear polarization direction of the transmitted signal.

The second and third resonant cavity pairs l2a-l2b and 1341-13,respectively, may also be resonant at the transmitted frequency butcoupled to the waveguide 10 through circular iris pairs 15a15b and16a-16b, respectively, in order to couple and therefore reject amultiplicity of waveguide modes. This is required because spuriousresponses of the transmitted frequency in the dominant TE. mode, causedby imperfections in the waveguide or existent in the signal at or priorto its launching, develop and propagate at higher modes, particularly THTE TM and TM, if that frequency lies above the waveguide cut offfrequency of these higher modes. The latter condition is common, sincethe next higher mode is only approximately 3l percent above the dominantmode. Spurious circular polarization in the dominant TE, is alsorejected, due to the positioning of the second and third resonant cavitypairs 120-12, and Ilia-13b, respectively, at 45 angle with respect tothe first resonant cavity pair Ila-1 lb. Due to the simultaneouspositioning of the second and third resonant cavity pairs -12 andl3a-13b, respectively, at 45 angle with respect to the polarization ofthe transmitted frequency in the dominant mode TE any coupling of thatfrequency in that polarization, and in the latter mode is negligible.

If the rejection of such a filter as shown and described herein is notsufiicient for any particular application, several such filters can beused in succession. The distance between such filters along thewaveguide 10 axis may preferably be an odd multiple of a quarterwavelength of the frequency transmitted in the waveguide 10.

What is claimed is:

1. A waveguide filter comprising:

an axially symmetrical waveguide;

at least three pairs of resonant cavities, diametrically disposed onsaid waveguide at angles which are integral multiplier of 45 withrespect to each other, in a plane orthogonal to the waveguide axis;

each one pair of resonant cavities being resonant at substantially thesame frequency; and

a plurality of irises in the wall of the waveguide for inductivelycoupling each of said pairs of resonant cavities to the waveguide toreject a plurality of predetermined modes of waveguide propagation.

2. A waveguide filter comprising:

an axially symmetrical waveguide;

first, second and third pairs of resonant cavities, each pair beingdiametrically disposed on said waveguide in a plane orthogonal to thewaveguide axis, the second and the third pairs of resonant cavitiesbeing disposed at 45 with respect to the first pair of resonantcavities, one on either side thereof; rectangular irises for inductivelycoupling said first pair of resonant cavities to the waveguide;

circular irises for coupling said second and third pairs of resonantcavities to the waveguide; and

said first, second and third pairs of resonant cavities being resonantat substantially the same frequency, whereby a plurality ofpredetermined modes of waveguide propagation are rejected by saidfilter.

l l i III '0'

1. A waveguide filter comprising: an axially symmetrical waveguide; atleast three pairs of resonant cavities, diametrically disposed on saidwaveguide at angles which are integral multiplier of 45* with respect toeach other, in a plane orthogonal to the waveguide axis; each one pairof resonant cavities being resonant at substantially the same frequency;and a plurality of irises in the wall of the waveguide for inductivelycoupling each of said pairs of resonant cavities to the waveguide toreject a plurality of predetermined modes of waveguide propagation.
 2. Awaveguide filter comprising: an axially symmetrical waveguide; first,second and third pairs of resonant cavities, each pair beingdiametrically disposed on said waveguide in a plane orthogonal to thewaveguide axis, the second and the third pairs of resonant cavitiesbeing disposed at 45* with respect to the first pair of resonantcavities, one on either side thereof; rectangular irises for inductivelycoupling said first pair of resonant cavities to the waveguide; circularirises for coupling said second and third pairs of resonant cavities tothe waveguide; and said first, second and third pairs of resonantcavities being resonant at substantially the same frequency, whereby aplurality of predetermined modes of waveguide propagation are rejectedby said filter.